Note: Descriptions are shown in the official language in which they were submitted.
04~
U-Wp-2805 Wacker
DENSE SHAPED ARTICLES CONSISTING OF
POLYCRYSTALLINE HEXAGONAL BORON NITRIDE AND
PROCESS FOR THEIR MANUFACTURE BY ISOSTATIC HOT PRESSING
__ .
Shaped articles consisting oE polycrystalline hexagonal
boron nitride are known. They are characterized by a combina-
tion of valuable properties such as high thermal conductivity,
low electrical conductivity, good mechanical workability, high
resistance to corrosion and to thermal shock. The articles show
poor wettability by molten metals such as aluminiu~, copper and
zinc; by non-metals such as silicon and boron and by non-metal-
lic compounds such as glass and cryolite. Because of their
properties, articles of polycrystalline hexagonal boron nitride
are important, for example~ as materials in the electronics
industry and as crucible materials.
Background of the Invention
!
The manufacture of shaped articles of this type is diEfi-
cult and expensive. It is virtually impossible by prior art
methods to densify pure boron nitride powder to a high degree
without the concomitant use oE sintering aids. The sintering
aids make it impossible to provide shaped articles having a
density approaching the density of pure boron ni~ride (2~27
g/cm3).
The best results known hitherto with respect to the
highest possible densification are achieved by hot die pressin~
at temperatures of from 1700 to 2200C at a die pressure of from
approximately 10 to 30 MPa, boron nitride powder containing bor-
ic oxide having proved especially useful. Owing to the boric
--1--
oxide content, the shaped articles thus obtained have poor high-
temperature properties.
To improve the high temperature properties, the oxygen
content of the boron nitride shaped articles manufactured by
hot-die-pressing is reduced by washing out the boric oxide after
sintering at Erorn 1600 to 2100C (see U.S. Patent 3,660,027 and
U.S. Patent 3,73~,997, corresponding to DE~AS 20 21 952). The
process, is unusually expensive and also is not useful for the
treatment of relatively large shaped articles. Washing out the
boric oxide to reduce the boric oxide content of relatively
small shaped articles requires up to 44 days.
Regardless of the type of sintering aids used, shaped
articles manufactured from hexagonal boron nitride powder by
hot-die-pressing exhibit a directional grain growth and an
anisotropic microstructure due to the biaxial application of
pressure at a high temperature so that the properties of the
articles are direction-dependent. The degree of anisotropy
increases as higher degrees of densification are achieved.
Furthermore, it is generally known that, in addition to
the customary hot-die-pressing using biaxial application of
pressure, isostatic hot-pressing, with multidirectional appli-
cation of pressure using an inert gas pressure-transfer medium
can also be usedO In isostatic hot-pressing, it is necessary to
provide the powder to be densified or the article preshaped
there~rom having open pores that is, having pores open to the
surface, with a gas-tight casing to prevent the gas used as a
pressure-transfer medium from penetratinq into the article and
interfering with densification.
Suitable casings for isostatic hot pressiny of boron
nitride powder having an excess of boron or nitrogen, are made
of titanium or zirconium~ The iront chromium or nickel casings
--2--
~ ~4~
customarily used for isostatic hot-pressing form, with the
excess boron or nitrcgen, compositions which have melting points
that are considerably lower than the melting points of the metal
casing material. The casings become molten and ~o longer gas-
tight and are penetrated by the pressure-transfer medium. It
should therefore be possible to subject boron nitride powders of
the type mentioned to the isostatic hot-pressing process at
temperatures of from 1200 to 1600C under a pressure of from 20
to 300 MPa, but it must be pointed out expressly that under
those conditions, boron nitride articles having high density
have not been obtained (see U.S. Patent 4,007,251).
Instead of the metal casings, preshaped articles may also
be placed in prefabricated glass casings and the space between
casing and article can be filled additionally with a glass
powder that has a higher softening temperature than that of the
casing itself (see Great Britain Patent Specification 1,529,g66
correspondiny to DE-PS ~6 01 294).
Furthermore, some experimental work is known relating to
the preparation of binder~free polygranular BN shapes from the
technical report "Development and Evaluation of Hot Isostatic-
ally compacted Boron Nitride" of M.C. Brockway et al, AD 709 620
of July 1970, Battelle Memorial Institute, Columbus Laboratory,
wherein commercial BN powders, purified by means of different
purification techniques, were hydrostatically cold-pressed to
form green bodies, which were then inserted into tantalum con-
tainers, sealed vacuum-tight by electron beam welding, and hot
isostatically compacted at temperatures of from 1650C to 2480C
under pressures of 207 MPa (30,000 p.s.i.) resp. 103 MPa (15,000
p.s.i.) for 1 to 3 hours.
It can be seen from the discussion of the experimental
results that the achieved densities were in the range of commer-
_~_
4 ~L
cial hot-die-pressed BN bodies. The data did not show a general
increase in density with temperature, presumably due to gross
grain growth at high temperatures. It was also sta~ed that im-
proved purity of the starting powders did not improve the prop-
erties such as density and mechanical strength, although the
oxygen contents were below the limit of the best commercial hot-
die-pressed BN, which usually is about 1 percent. Furthermorer
it was stated that the samples were not completely isotropic~
i.eD, that obviously a directional grain growth could not be
suppressed despite the multidirectional applica~ion of pressure.
X-ray diffraction data disclosed a subs~antial amount of aniso-
tropy in the articles which appeared to decrease as the length
to diameter ratio of the article increased.
The critical parameters; which are necessary ~or manufac-
turing shaped articles consisting of boron nitride by means of
the isostatically hot pressing procedure to obtain improved
properties namely, high density combined with a substantially
isotropic microstructure, were not noted by the authors of this
report.
The object of the invention is therefore to provide dense
shaped articles consisting of boron nitride having improved
properties, which are manufactured in a simple manner without
expensive after-treatment, by densification of pure boron
nitride powder without the concomitant use of sintering aidsO
Brief Summary of the Invention
The shaped articles accoring to the invention, have a
density of at least 95~ of the theoretical density (abbreviated
hereinafter as %TD) of boron nitride (BN), and consist of poly-
crystalllne hexagonal boron nitride having an isotropic micro~
structure. The articles are manuEactured from pure boron
--4--
g4~
nitride powder having a boric oxide content of not more than
1.0~ by weight, by isostatic hot-pressing in a vacuum-tigh~
cdsing at a temperature of from about 1200 to 1500C under a
pressure of from about 50 to 300 MPa using an inert gas as the
pressure-transfer medium.
Detailed Description of the Invention
.
The shaped articles according to the invention are prefer-
ably manufactured rom fine powders consisting of hexagonal
boron nitride having a specific surface area within the range of
from about 5 to 30 m /g (measured according to the BET method
which relates to the particle size), having a purity of at least
98.5~ by weight which means that the sum of the analyses o bor-
an and nitrogen is at least 98.5% by weight~ The free boric
oxide content of the powders is, according to the definition,
not greater than about 1.0~ by weight. Metallic impurities
totalling up to about 0.2~ by weight can be tolerated. The
difference in composition up to 100% by weight is due for the
most part, to oxygen in the form of adhering boron oxynitrides.
Commercial boron nitride powders from any desired source,
that have been purified by customary methods such as heating to
approximately 1800C and subsequently washing with methanol or
water can be used in forming the articles of the present
invention.
The shaped articles according to the invention can be
manufactured by filling prefabricated casings of a desired shape
with the pure boron nitride powder per se; the BN in the casings
can then be compacted by vibration. Because of it5 low bulk
density, the boron nitride powder can be advantageously granula-
ted with small amounts of a temporary binder such as polyvinyl
alcohol. The casing, together with its contents, are then
--5--
~.~$~
e~acuated and sealed gas-tight. If a temporary binde~ is utili-
~ed, it must be removed from the BN preferably by heat and
vacuum before sealing the casing.
The boron nitride powder can, however, be preshaped into
green bodies having an open pore structure that is, having pores
open to the surface and can then be sealed under vacuum in a
gas-tight casing. The shaping can be carried out by convention-
al methods as, for example, by die pressing or isostatic press-
ing. The concomitant use of a temporary binder is, in this
case, generally not necessary. If desired, however, small
amounts of a temporary binder can be used~ Examples of tempor-
ary binders are polyvinyl alcohol and camphor which can be used
in amounts of up to about 3~ by weight, based on the weight oE
the boron nitride powder. It is critical that the binders
decompose leaving virtually no residue at temperatures of less
than about 1000C or are present in such low concentrations that
the decomposition residue in the form of free carbon does not
exceed about 0.1~ by weight, based on the boron nitrideO Al~
though relatively large amounts of free carbon do not interfere
with the densification process during the subsequent isostatic
hot-pressing, they may cause a discoloration of the end product
which is usually white, as follows from the known designation of
boron nitride as "white graphite".
After being shaped, the green bodies preferably have a
theoretical density of at least about 50% and most preferably
about 60~ TD and higher. Subsequently, the green bodies are
preferably subjected to a thermal treatment by heating to from
about 300 to about 1000C, most preferably, from about 400 to
about 700C, before they are provided with a gas-tight casing,
in order to ensure that during hot isostatic densification,
neither gaseous decomposition products from the binders nor
water vapor interferes with the densification process or damages
the caslng.
The material used for the casings must be able to be
sealed gas-tight, and must be plastically deformable at the
relatively low densification temperatures of from abou~ 1200 to
about 1500C reguired for boron nitride. Metals such as iron,
chromium or nickel; metal alloys such as steel, and glasses or
vitreous compositions which are plastically deformable within
the densification temperature range can be used.
When using boron nitride powder per se, prefabricated
casings or capsules are required. Steel casings which are easy
to handle and are available at a favorable cost, have proved to
be especially useful. Prefabricated casings can also be used
with preshaped green bodies. In the case of preshaped green
bodies, the gas-tight casing can, however, be produced by direct
coating as, for example, by electroless wet deposi~ion of a
metal layer or by application of a vitreous composition which
can subsjequently be fused or sintered to form the gas-ti.gh~
casing.
The encased samples are placed in a high-pressure auto-
clave and heated to the required densification temperature of at
least about 1200C. In the process, it is advantageous to con-
trol pressure and temperature separately that is, to increase
the gas pressure when the casing material begins to plastically
deform under the pressure and temperature conditions. Preer-
ably, argon or nitrogen are used as inert gases for ~he trans~er
of pressure. The pressure applied lies preferably within the
range of from about 100 to about 200 MPa and is reached by
gradual increase in pressure ~hen the final temperature used in
each case, which is preferably at least about 1250C, has been
reached. The optimum temperature in each case is dependent on
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~194~
the fineness and purity of the boron nitride powder used. The
optimum temperature, and in any case a temperature of about
1500C, should r.ot be exceeded in order to ensure that boron
oxynitrides optionally present are not decomposed to form boric
oxide.
After the pressure and temperature have been reduced, the
cooled articles are removed from the high-pressure autoclave and
freed from the casings as, for example, by twisting off metal
casings or by sand blasting glass casings.
Depending on the pressure applied, and the temperature and
the dwell time under these conditions, the shaped articles
manufactured by the present process have a density of at least
95~ TD~ preferably at least 99%TD. Owing to the multidirection
al application of pressure and the relatively low pressing
temperature, the articles produced by the present process, even
at high degrees of densification, have a homogeneous, isotropic
microstructure and their properties are no longer direction-
dependent but are substantially equal in all directions.
The independence of the physical properties on direction
is confirmed by the substantially identical values measured on
specimens which were machined from the hot isostatic pressed
boron nitride articles of the present invention. No grain
growth was observed under the conditions of the process of the
present invention that is, the grain sizes in the microstructure
of the finished shaped article correspond virtually to ~he
particle sizes of the boron nitride powder used. Articles with
a flexural streng~h (mechanical strength) greater than 50 N/mm2
are obtained by the present process. The articles of the
present invention maintain their isotropic microstructure and
mechanical strength at relatively high temperatures. The shaped
articles have high thermal conductivity and a low E-modulus and
--8--
are thereEore highly resistant to thermal shock.
The shaped articles according to the invention consisting
of polycrystalline hexagonal boron nitride are characterized by
superior physical properties as compared with articles manufac-
tured with the concomitant use o sintering aids, according to
the conventional hot-pressing process and/or expensive after-
treatment, and also better physical properties than articles
prepared without sintering aids by isostatic hot pressing at
relatively high pressing temperatures.
The articles of the present invention can be manufactured
in a relatively simple manner, since the shape and the size of
the article are neither limited by the adequately known limited
shaping possibilities in a customary hot-die-press using die
pressure, nor by the expensive and time-consuming encapsulating
technique according to isostatic hot pressing at relatively
high pressing temperatures which requires the use of tantalum
containers. The tantalum containers must generally be sealed
gas-tight by techniques such as electron-beam welding. The
shaped articles according to the invention can be manufactured
for example in a h gh-pressure autoclave, having a useful volume
of greater than lm , as relatively large articles in block form
using simple steel casings which can be sealed gas tight in a
simple manner without the need for complex sealing techniques.
The large articles can then be machined independently of
direction to form relatively small articles of a desired shape.
Although it is known tha~ by the isostatic hot-pressing
process, articles consisting of pulverulent materials provided
with a gas-tight casing can be densified, it is unexpected that,
in the case of boron nitride, this can be achieved without the
concomitant use of sintering aids at temperatures of less than
about 1500C using simple steel casings which hitherto had no~
._9 _
been considered possible.
The inventlon is explained in detail with reference to the
following examples.
Example _
A boron nitride powder having the following analysis
% by weight
B 43.3
N2 55.7
Total 2 0.8
B203 0.07
Ca
and a specific surface area of 14 m2/g was granulated using 0.3%
by weight of polyvinyl alcohol and shaken on a vibrating table
into a steel capsule (St 37) having a diameter of 120 mm and a
height of 150 mm. The compacted apparent density oE the granu-
late was approximately 0.8 g/cm3O A lid provided with an
extraction connection piece was then welded onto the capsule,
which was then heated in a Eurnace to 550C. At the same time,
the capsule was evacuated, through the extraction connection
piece, using a vacuum pump. The end of the degassing operation
was determined by a vacuum indicator. The extraction connection
piece was squeezed together vacuum-tight with hydraulic nippers,
cut off above the squeezed point and secured with a weld seam.
The evacuated capsule filled with boron ni~ride was placed in an
isostatic hot press and densified at 1450C under an argon gas
pressure of 200 ~lPa and a dwell time of 180 min.
After cooling, the steel capsule was removed by cutting it
open and uncciling it from the boron nitride cylinder. The
-10--
4~
density of the shaped article was 2.24 g/cm (corresponding to
99.1%TD, based on the theoretical density of the powder of 2.26
g/cm ). The structure was finely crystallineO No grain growth
with respect to the starting powder was observed.
The following properties were determined by testing
specimens which were cut in the axial or in the radial direc-
tion, respectively, Erom the hot isostatic pressed boron nitride
cylinder. The results are set forth in the following Table 1:
Tahle 1
factor
properties axial radial axial/radial
flexural strength
in N/mm2 74.5 73.3 1.016
E-modulus in
N/mm2 62 400 64 700 0.964
coefficient of
thermal expansion
in K 1 (25~1000C) 2.92 10 6 2.78 10 6 1~050
Knoop~hardness
HK 100 54 52 1.038
HK 25 62 61 1.016
~hermal conducti-
vity in W/mk
300 K 74 72 1.027
600 ~ 52 51 1.019
lOOOK 39 39 1.000
--11--
o~
The flexural strength was measured according to the 4-point loading
(outer span 30 mm, inner span 15 rnm) using rectangular test bars of
the size 2 x 4 x 34 mm. The Knoop-hardness was determined by using
a load of 0.98 N (HK lO0) and 0.245 N ~HK 25). The values are
related to daN/mm~.
As can be seen from the substantially identical values meas-
ured on specimens which were cut in the axial or in the radial
direction, respectively, the properties were independent of the
direction.
The results shown in Table 2 were determined to be independ-
ent of the direction in the article.
Table 2
compression strength in N/mm 297.5
loss tangent ~ ~ 3 x 10 4
~average value of 20 measurements at lO0 kHz)
dielectric constant ~ 4.90
electrical resisting in ohm x cm 2020 x lO
., . .... _
The loss tangent ~ ~ was measured at a frequency of lO0
kHz; comparative measurements in the range of lO kHz to lO MHz did
not show significant dîfferences.
The dielectric constant was calculated from th~ capacity,
which was also measured at a frequency of 100 kHz.
Example 2
A boron nitride powder having the following analysis
~ by weight
B 43.2
-12-
~S4~4~
N2 55.4
total 2 1.3
B203 - 9
Ca 0.05
and having a specific surface area of 25 m2/g was isostatically
pressed in a polyvinyl chloride casing at 400 MPa liquid pressure
without pressing additives to form a green body having a diameter
of 230 mm and a height of 300 mm. The density of the green body
was 1.76 g/cm3 (corresponding to 79%TD~. The polyvinyl chloride
casing was removed. The green body was placed in a suitable steel
capsule (St 37j and a lid provided with an extraction connection
piece was welded on. The heating and sealing of the capsule was
carried out as described in Example 1. The encased sample was hot
isostatically pressed at 1400C under an argon pressure oE lS0 MPa
and a dwell time of 120 min. Af ter densification, the casing was
removed by cutting it open and uncoiling it. The density of the
shaped article obtained was 2.21 g/cm3 ~corresponding to 99.1~TD,
based on the theoretical density of the powder used of 2.23 g/cm3).
The microstructure was finely crystalline.
Example 3
The boron nitride powder of Example 2 was isostatically
cold-pressed to form a cylinder having a diameter of 60 mm and a
height of 120 mm and was placed in a silica-glass capsule. The
cold-pressing pressure, the density of the pressed article and t~e
heating conditions corresponded to the conditions used in Example
2, After degassing, the extrac~ion connection piece was sealed
vacuum-tight. The glass-encased sample was ho~ isostati~ally
pressed at 1500C, 200 MPa argon pressure for 180 minutes dwell
time. After densification, the glass casing was removed. The
-13-
density of the sh~ped article obtained was 2.23 9/cm3 (corres-
ponding to 100% TD~. The microstructure was finely crystalline.
